It is well-known to those in the field of flow measurement that the velocity of a fluid can be determined by a phenomenon known as a "von Karman vortex street" in which fluid vortices generated by a fluid flowing past a bluff body will pass downstream on alternate sides of the bluff body. The number of fluid vortices shed past the bluff body during a given time period is proportional to the velocity of a passing fluid.
Numerous flow meters have been developed over the years using the principal of a yon Karman vortex street. For instance, U.S. Pat. No. 4,862,750, issued to the present inventor, discloses a fluid velocity meter adapted to span the full diameter of the pipe having dual force sensors disposed on opposite sides of a tail piece located downstream from the vortex generating element. The vortices which shed from the vortex generating element pass downstream and alternately impinge on each of the sensors in the tail piece to thereby produce an output signal related to the deformation of the sensors. The sensors are positioned in close proximity to one another in the tail piece and a filler material is provided between the sensors to permit independent lateral movement of the sensors relative to one another. This arrangement provides excellent measurement properties. However, one shortcoming of this arrangement is that the signals produced by the sensors in the tail piece are not completely independent of one another, i.e., the inward deformation of one sensor tends to cause the outward deformation of the other sensor, which tends to produce unwanted secondary signals and can complicate the signal processing of the flow meter.
U.S. Pat. No. 4,559,832 to Burlage et al. discloses a vortex shedding meter which employs a pair of piezoelectric force sensors disposed in a sensor chamber, with the sensors being separated by a rigid metallic portion. The vortex generating element, however, is not integral with the sensor carrying body but is separately provided and located upstream from the sensor carrying body.
Another flow meter, U.S. Pat. No. 3,587,312 to McMurtrie et al., discloses a differential sensor bluff body flow meter wherein a pair of sensors are integrally mounted on a bluff body which is removably mounted through an aperture in the pipe. The sensors are integrally formed as a part of the bluff body which provides the oscillatory fluid flow.
There are also other flow meters which sense the shedded vortices through the monitoring of a modulated ultra-sonic signal, for example, U.S. Pat. Nos. 3,797,309, 4,031,757 and 4,240,229. All of these aforementioned patents disclose sonic vortex sensors whereby a transmitted and reflected sonic signal is acted upon by the vortices generated past a strut element.
Despite the multitude of differently constructed fluid velocity meters, there is still a further need to provide an improved fluid velocity meter, and especially an improved flow meter which is of the insertion-type, i.e., a universal flow meter directly insertable into pipes or conduits with different diameters rather than being constructed in a casing having a pre-determined diameter to match the pipe next to which it is applied, i.e., the casing carrying the flow meter must be adapted to fit more or less flush between two joining pipe sections. Such insertion-type flow meters are advantageous since they eliminate the need to change the dimensions and construction of the flow meter and casing according to the diameter of the pipe to which they are applied. Thus, insertion-type flow meters are generally more cost effective since it is not necessary to purchase a different size flow meter for a given diameter. Moreover, such an insertion type flow meter is easily insertable and removable from a pipe for easy access for repairs, replacement or the like.
Not only is there a need to provide an improved insertion-type flow meter with the aforementioned advantages, but there has also been a need to eliminate the problems associated with vortex shedding flow meters heretofore which do not adequately compensate for flow disturbances which interfere with the measurement of the vortices and thereby producing inaccurate measurements of fluid velocity. In this regard, it is desirable to provide a flow meter that is capable of minimizing or compensating for unwanted fluid pulsations and vibrations that can occur within the conduit as well as other forms of fluid noise which tend to interfere with the accuracy and operation of the flow meter.
The invention of the present application addresses the aforesaid needs.